1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor chip bump for densely-packed mounting, and more particularly to a method for manufacturing a semiconductor chip bump wherein a photoresist layer is formed by repetition of a photolithography process on a region to obtain the semiconductor chip bump of a desired shape required for bonding internal leads in a tape automated band (TAB) package of a desired shape and improve reliability during bonding the internal leads of a lead frame in the TAB process.
2. Description of the Related Art
In TAB packages, a metal pattern serving as a lead frame and associated wiring is generally provided on an insulating film. The TAB package, in contrast to bonding wire packaging, is one kind of surface mount packaging technique that bonds the metal pattern formed on the insulating film to a bonding pad of a semiconductor chip using a physical "bump" composed of a conductive material. This method is widely used in hand-held calculators, liquid crystal displays (LCDs), computers, etc.
The TAB package has been developed as a slim TAB or small TAB package to obtain miniaturization and thinning of the resulting package.
FIG. 1 is a plan view showing one example of a TAB package using a general tape carrier.
Referring to FIG. 1, a tape carrier 10 applied to the TAB package is formed such that a metal layer attached on a base film 12 consisting of polyimide, polyester, polyester sulfone, polyparalic acid, etc. is subjected to a photolithography process to form internal leads 13 and external leads 14. Internal leads 13 are connected to a semiconductor chip 11.
The center of the base film 12 is punched to form a device aperture 15 to expose ends of the internal leads 13. A slot 16 is formed to expose one side of the external leads 14 for electrical connection to external pins.
A bump (not shown) is formed on the substrate adjacent to the lower end of the internal leads 13 by a thermo-compression method for electrical connection to the electrode pin of the semiconductor chip 11. In the TAB package, the internal leads 13, the bump, the upper surface of the semiconductor chip 11, etc. are shielded by a sealing resin (not shown).
Along with achieving high packing density in the semiconductor device, the fine pitch in the TAB package increases the number of leads and reduces the spacing between the leads and, in turn, the required spacing between the bumps on the semiconductor chip.
Such a semiconductor chip bump used in TAB packaging is formed by any conventional method, such as an electroplating process or an oxidation-reduction to deposit a metal, using a photoresist pattern on a pad of the semiconductor chip. The method for manufacturing the semiconductor chip bump will be described in detail below.
FIGS. 2A and 2B are schematic sectional views showing a method for manufacturing a semiconductor chip bump according to a conventional technique.
Referring to FIG. 2A, a metal line 24 such as a printed circuit line, is formed on a semiconductor substrate 21 via a wiring pattern formation process to establish a bonding pad, and a photoresist pattern 22 is formed to a thickness of approximately 20 .mu.m which exposes a barrier metal layer 26 formed on the bonding pad.
The bonding pad is provided by a sequentially-formed insulating layer 23, metal line 24, and passivation layer 25 on the semiconductor substrate 21. A predetermined portion of the metal line 24 in the bonding pad is exposed to form the barrier metal layer 26 on the metal line 24 and the passivation layer 25.
A general photolithography process for forming the photoresist pattern is performed as follows. A photoresist solution composed of a photosensitive material, resin, etc. is dissolved in a solvent and is coated evenly on a semiconductor substrate via a spin-coating. A soft baking is then carried out at a low temperature. Then, light is selectively irradiated through a pattern mask, and a development step is performed to form a photoresist pattern of a predetermined shape. At this time, an exposed or unexposed portion of the pattern mask is selectively removed in the developing process, using a weak alkaline developer solution having tetra-methyl-ammonium hydroxide (TMAH) as a main ingredient, thereby forming the photoresist pattern.
Referring to FIG. 2B, the photoresist pattern 22 of a predetermined shape formed on the semiconductor substrate 21 forms a bump 27 having a predetermined height on the portion of the barrier metal layer 26 exposed by the photoresist pattern 22. The photoresist pattern 22 is then removed.
The bump 27 must have sufficient height, (e.g., approximately 20 .mu.m), to prevent contact between the semiconductor substrate 21 and internal leads 13 during the process of bonding the internal leads of the TAB package.
However, in the conventional method for manufacturing the semiconductor chip bump, a bump having a desired height is formed using a photoresist pattern formed via a single photolithography process as a pattern mask. Because a single photolithography process is used, the thickness of the photoresist layer is limited to about 20 .mu.m. This is the distance over which light can travel while still inhibiting an increase in the aspect ratio (a ratio of height to diameter) of the bump beyond a certain degree. Therefore, the aspect ratio cannot be selectively adjusted in the conventional method.
As the height of the conventional semiconductor chip bump increases, the difference between the upper and lower diameters of the photoresist pattern increases in conjunction with the characteristics of the photolithography process (note FIG. 2A). Thus, the lower diameter of the bump becomes smaller than the upper diameter thereof, thereby concentrating force onto the bonding pad when bonding the bump to the internal leads, thereby causing cracks in the bonding pad.
Because high packing density reduces the spacing between the bumps during the process of bonding the internal leads to respective bumps, the upper larger diameter portion of adjacent bumps can contact one another.
Also, the photoresist pattern on the lower portion of the bump is frequently not completely removed by the process of removing the photoresist pattern. An additional process for removing the remaining photoresist pattern must therefore be performed, which makes the manufacturing process more complicated.
Moreover, if the edge of the upper surface in the bump is higher than that of the center portion thereof, contact failure with the internal lead can occur due to inconsistent contact between the leads and the bump. In order to prevent this, a mushroom-shaped bump may be formed, which, however, has a bump higher than the photoresist pattern. Therefore, contact failure with the internal leads can be prevented, but cracks in the bonding pad can occur or the bumps can contact each other.